Fabricating high-purity graphite disk electrodes as a cost-effective alternative in fundamental electrochemistry research

Graphite electrodes offer remarkable electrochemical properties, emerging as a viable alternative to glassy carbon (GCE) and other carbon-based electrodes for fundamental electrochemistry research. We report the fabrication and characterization of high-purity graphite disk electrodes (GDEs), made from cost-effective materials and a solvent-free methodology employing readily available laboratory equipment. Analysis of their physical properties via SEM, EDX and XPS reveals no metallic interferences and a notably high porosity, emphasizing their potential. The electrochemical performances of GDEs were found to be comparable to those of GCE. Immobilization of peptides and enzymes, both via covalent coupling and surface adsorption, was used to explore potential applications of GDEs in bioelectrochemistry. Enzyme activity could be addressed both via direct electron transfer and mediated electron transfer mechanism. These results highlight the interesting properties of our GDEs and make them a low-cost alternative to other carbon-based electrodes, with potential for future real-world applications.

All samples have been fitted with the equivalent circuit above, except for the 15% PE GDE, for which a third Cole element (CPE3 + R3) was required for proper fitting.The resistance Rs originates from the overall cell resistance and is not relevant to the current analysis.The Cole elements consist of a constant phase element, CPE, and a resistance R in parallel.The two fitting parameters for the CPE are the capacitance factor (CPE-T) and the phase factor (CPE-P, ranging from 0 to 1).The results for all electrodes are summarized in Table S2, divided based on circuit element.
Every Cole element represents a charge transfer/accumulation step.The first element accounts for charge accumulation in the double-layer, and, for GCE and 25% PE, it can be fitted with a phase factor of ca.0.9.In contrast, in the case of the pure graphite GDE, the phase factor becomes close to 0.5, and the Nyquist plot appears as a straight line instead of a semicircle (i.e. a Warburg impedance behavior).
In absence of PE, in fact, the electrolyte can penetrate the pores of the material, in which the diffusion of charges is presumably slow.Hence, diffusion processes dominate the impedance signal.The 15% PE GDE exhibits a mixed behavior and would require a more elaborated fit, which goes beyond the scope of our current analysis.
Moreover, the values of R for the first Cole element are all quite large, as expected when no electron transfer occurs (no faradaic processes involved).
In our material, it is reasonable to attribute the second element to the charge hopping between the graphite particles or to the charge transfer between the electrode contacts.This element will contribute to the impedance at high frequency, with negligible values of CPE and R.
Figure S3.Custom-designed electrochemical cell under operating conditions.

Figure S4 .
Figure S4.XPS measurements of 25% PE GDE after polishing.High-resolution spectra of the (a) C 1s and (b) O 1s regions.

Figure S7 .Figure S8 .
Figure S7.Background traces in phosphate buffer upon reduction and after Ar bubbling, performed right after reduction.

Figure S10 .
Figure S10.Plots of the linear dependence of the peak currents to the square root of the scan rate, comparing 15% PE, 25% PE GDEs and GCE.

Figure S11 .Figure
Figure S11.Double-layer capacitance measurement for (a) GCE, (b) 15% PE GDE, (c) 25% PE GDE; (d) linear dependence of the anodic current at 0.15 V to the scan rate for all electrodes in MES buffer.

Figure S13 .Figure
Figure S13.Comparison of dropcasting of BOD on GDE and GCE.

Table S2 .
Fitting results for EIS analysis.